The Marine Corps' fighting ability is undermined by the more than 30,000 active Marines and veterans involved in a private Facebook group being investigated for allegedly posting explicit photos of women in the military without their consent, a former Navy secretary told CNN on Friday.

"There's no good excuse," Ray Mabus said on CNN's New Day. "The Marine motto is 'semper fidelis,' always faithful, and what these Marines did was to be unfaithful to their fellow Marines."

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"Marines look after each other, Marines have each other's back. Look at what the commandant of the Marine Corps said about this, this makes us a less effective fighting force," Mabus said, referring to a video statement posted Tuesday by Gen. Robert Neller.

Neller condemned those involved in the Facebook group, saying their behavior is not of "true warriors or war fighters" and that "cohesion and trust within a unit, and between marines, is vital to success on the battlefield."

Mabus, who worked to empower women in the military and advocated putting them in combat roles, echoed Neller's sentiment. He said the Facebook group, dubbed "Marines United," and its postings undermine the fighting ability of the Marines.

"That's what's at stake here. It makes Marines less able to fight," he told New Day's Chris Cuomo.

The investigation into the Facebook group should look into how it developed and whether anyone in the chain of command knew about it, Mabus said.

Four branches of the military are looking into the posting of nude photos of what appear to be female service members on various websites, a Pentagon official told CNN.

"These Marines, these veterans, that have participated in this have attacked fellow Marines, and that is inexcusable in a fighting force, and particularly in the Marines, who are so special." Mabus said. "Every Marine has to know that every other Marine has their back."